One representative example of an image-forming apparatus using the electrophotographic method that transfers a toner image formed on an image carrier to a transfer belt device is a full color image-forming apparatus that forms a multiple-colored image (e.g., a full-color image). In a full-color image-forming apparatus, toner images of each color that are sequentially formed on an image carrier are transferred sequentially, one on top of another, to a transfer belt such that they are superimposed onto the transfer belt, whereupon the overlapping toner images are finally transferred to a recording medium.
Specifically, in a full-color image-forming apparatus, toner images comprising such colors as cyan, yellow, magenta and black are sequentially formed on a photoreceptor drum through an electrophotographic process. As each color toner image is formed, it is transferred onto the transfer belt in a first transfer, and after toner images of each color are transferred onto the transfer belt and overlapping toner images are formed on the transfer belt, the overlapping toner images are then transferred onto the recording medium in a second transfer.
However, when this type of transfer belt device is used, the following problems arise.
If the transfer belt has the proper tension and is conveyed smoothly, and the conveyance speed is a constant speed essentially the same as that of the movement of the surface of the image carrier, the toner image on the image carrier is correctly transferred onto the transfer belt. However, in actuality the belt conveyance speed fluctuates and slackness may occur in the belt. If this is the case, and particularly if there is slackness in the section of the belt downstream from the transfer area where the transfer roller is in terms of the direction of belt movement, the toner image being transferred to the belt, or the toner images already transferred to the belt, become distorted or smudged.
The fluctuation in transfer belt conveyance speed or slackness in the transfer belt is caused mainly by its slipping off the belt drive roller or by fluctuations in the load on the belt. Slipping of the transfer belt occurs if there is insufficient friction between the surface of the drive roller and the transfer belt, even if the drive roller is driven at a prescribed speed. Fluctuation in the load on the belt may occur when a cleaning member is caused to come into contact with the belt at a location upstream from the transfer area in terms of the direction of belt movement, for example, in order to eliminate any toner that did not get transferred from the transfer belt to the recording medium during the second transfer and remained on the belt. Alternatively, such a fluctuation in load may occur when the second transfer roller is pressed onto the transfer belt via the recording medium in order to transfer onto the recording medium the toner images that were previously transferred onto the transfer belt.
Normally, in order to increase the speed of image formation, when the second transfer roller and cleaning member are pressed onto the belt in this way, the subsequent first transfer is also carried out at the same time. The result is a defective first transfer of the toner image arising due to a fluctuation in the load on the belt.
The slipping of the transfer belt referred to above is generally thought to be resolvable by forming the driver roller using an elastic material that increases the amount of friction between the transfer belt and the drive roller. However, if the drive roller were simply formed of an elastic material, when the load on the transfer belt changed, this fluctuation in load would be transmitted to the outer layer of the drive roller, which would become deformed, changing the roller's outer diameter and causing the belt conveyance speed to fluctuate.
Accordingly, the object of the present invention is to provide a transfer belt device that, simply through adjustment of the rotational speed of the belt drive roller, can eliminate fluctuations in belt conveyance speed and problematic slackness in the section of the transfer belt downstream from the area at which the toner image is transferred onto the transfer belt, in terms of the direction of belt movement, and can thereby transfer the toner image on the image carrier onto the transfer belt without problems such as distortion or smudging.